I have a cadence monitor, but i rarely use it. I am a natural spinner, so that isn't hard for me. But, I do find it hard to average 90 on the flats. I might be doing 90-100 at times, but my usual average cadence is about 75-85. Of course, every ride I do ends with a monster hill, so the average cadence is down. I tend to use the easiest gear I can, where i am still feeling resistance. Last fall I started using the big ring a lot more on flats. While it allows me to get my speed up while I draft my husband, it immediately hurts my knees. I could probably ride and NEVER use the big ring because of this, but I make myself.
I find that if I leave my computer on cadence while I am riding, I obsess on it and then it's not fun. Same with my HR monitor. I use them both for indoor training, but once I get outside, I want to enjoy myself. I seem to be able to guess quite accurately when my av. cadence is up or down from the usual.